Labelled histogram

Plots the histogram for the observations, but grouped by labels.

Imagine that you have a set of observations that have been properly labelled. It should be interesting not only to analyze the data pdf but also to study the weigth of each class (observations' labels) to each one of the histogram bins. In this way I have developed a version of HIST, that is called LHIST. This new function plots the histogram for the observations grouping them in M bins. In addition, it accounts for the different elements that are present in each bin grouping them by LABELS.